Published on Web 10/02/2004
Dirhodium(II) Caprolactamate: An Exceptional Catalyst for Allylic Oxidation Arthur J. Catino, Raymond E. Forslund, and Michael P. Doyle* Department of Chemistry and Biochemistry, UniVersity of Maryland, College Park, Maryland 20742 Received August 3, 2004; E-mail:
[email protected] Allylic oxidation holds a venerable position in organic synthesis. The regioselective functionalization of an allylic C-H bond with oxygen is a value-added operation yielding a wide range of synthetically useful products.1,2 Particularly important is the direct synthesis of enones and enediones from the oxidation of readily available alkenes.3 Traditional methods typically employ stoichiometric metal oxidants.4 Transition metal catalyzed processes in conjunction with a terminal oxidant offer a promising alternative, but limitations arise from high catalyst loading, lengthy reaction times, harsh conditions, and/or poor selectivity.5 Herein we describe a new allylic oxidation protocol using dirhodium catalysis that is mild and selective and operates with unprecedented turnover number and frequency. Dirhodium(II) complexes, specifically Rh2(OAc)4 and Rh2(pfb)4 (Figure 1), have been reported to have limited activity for allylic
found that under the same conditions 1 mol % Rh2(cap)4 provided 2 in 33% yield in only a few minutes.15 Considering literature precedence for the intermediacy of allylic tert-butyl peroxy ethers16 along with their facile conversion to carbonyl compounds with base,17 addition of K2CO3 (50 mol %) to the reaction mixture greatly improved yield to 80% without compromising reaction time.18 Upon further examination, we were able to reduce catalyst loading to only 0.1 mol % Rh2(cap)4 (substrate/catalyst loading ) 1000) producing enedione 2 in 78% yield in just 1 h. Further optimization showed a considerable dependence on the amount of terminal Table 1. Rh2(cap)4-Catalyzed Allylic Oxidation of Olefins19
Figure 1. Dirhodium(II) catalysts.
oxidation of olefins with tert-butyl hydroperoxide (TBHP).6 With poor yields that preclude wide application, little has been done to address the activation of peroxide by dirhodium.7,8 Generally speaking, the most effective catalysts for allylic oxidation, as pointed out by Kochi, are those metals that readily undergo 1-electron redox processes, e.g., Fe2+ / Fe3+, Cu1+ / Cu2+, and Co2+ / Co3+.9 With this in mind, we considered bench-stable and readily prepared dirhodium(II) caprolactamate (Rh2(cap)4, (vide infra).10 Cyclic voltammetry data for Rh2(cap)4 obtained by Ren showed a reversible oxidation at 55 mV (in CH3CN, vs Ag/AgCl) corresponding to the Rh24+/Rh25+ redox couple.11 Moreover, Rh2(cap)4 readily underwent a 1-electron oxidation (E1/2 ) 11 mV), in contrast to Rh2(OAc)4 and Rh2(pfb)4 (E1/2 ) 1170 and >1800 mV, respectively).12 This striking data revealed that Rh2(cap)4 could access and transverse the Rh24+/Rh25+ oxidation states and compelled us to examine its ability as a catalyst for allylic oxidation. We initiated our study by examining the allylic oxidation of 1-acetylcylcohexene 1 (eq 1).13 As expected, 1 mol % Rh2(OAc)4
or Rh2(pfb)4 with anhydrous TBHP14 (5 equiv) in CH2Cl2 after 12 h yielded only trace amounts of enedione 2. On the other hand, we 13622
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a Isolated yield after chromatography. b Diminished yield due to product volatility. c Reaction performed on a 1.00-g scale. d Catalyst added in two portions (0.5 mol % catalyst and 5 equiv of TBHP at the start, and another 0.5 mol % catalyst and 5 equiv of TBHP midway through the indicated time). e Reaction performed at 40 °C.
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oxidant, as product yield diminished with TBHP below three equivalents consistent with the stoichiometry of the reaction.18 Screening various solvents showed that CH2Cl2 was superior, and no reaction was observed in THF or acetonitrile.18 Using these optimized conditions, we examined the allylic oxidation of representative olefins (Table 1).19 Most olefins were rapidly converted to enones and enediones in 1 h with only 0.1 mol % Rh2(cap)4. Complete selectivity was observed in all cases, yielding only the products indicated. Noteworthy is the selectivity observed for 1-methylcyclohexene (entry 6) and 3-methycyclohexene (entry 7), which were both converted to seudenone, an insect pheromone isolated from the Douglas-fir beetle.20 Overall, the reaction is operationally simple to perform while being tolerant of air and atmospheric moisture. Mechanistically, extensive investigations have shown that Rh2(cap)4 undergoes a 1-electron oxidation in the presence of TBHP to form a Rh25+ species tentatively assigned as 4, which can be converted under the reaction conditions to dirhodium peroxyether complex 5 (Scheme 1). Evidence for this oxidative transformation includes a dramatic color change from light blue to deep red in CH2Cl2. The UV-visible spectrum of the catalyst upon addition of TBHP revealed a low-energy adsorption at 974 nm (δ-δ* transitions) consistent with a mixed-valent dinuclear metal species.21,22 In addition, the product of the reaction of Rh2(cap)4 and TBHP was isolated and analyzed by 1H NMR despite its paramagnetic nature and showed a new signal at 1.24 ppm.23 We believe these data support the formation of dirhodium tert-butyl peroxyether complex 5. Furthermore, evolution of oxygen during the course of the reaction was observed and is consistent with the formation of tertbutyl peroxy radical 3, which is known to dimerize to di-tertbutyltetraoxide.24 More importantly, though, 3 is capable of selective hydrogen atom abstraction to produce radical 6.25 Ligand transfer of the metal-bound peroxide to the carbon-centered radical forms the mixed peroxide 7 and regenerates the catalyst (as Rh24+). This proposed sequence thus constitutes a redox chain catalytic cycle.26 Finally, rapid decomposition of 7, which we have isolated as a product from the reaction, yields the enone or enedione as shown.27 Scheme 1. Mechanistic Proposal for Allylic Oxidation Catalyzed by Rh2(cap)4
In summary, we have developed a novel catalytic allylic oxidation protocol based on dirhodium. The unique reactivity of Rh2(cap)4 emanates from its ability to undergo facile redox chemistry, namely Rh24+ / Rh25+. Further studies will be aimed at accessing the active oxidant and developing new applications including enantioselective processes that involve higher-valent dirhodium intermediates.
Acknowledgment. We are grateful to the NSF and the NIH (GM-46503) for their generous support. Special thanks to Prof. Tong Ren for alerting us to the comparative electrochemical data. Supporting Information Available: Experimental procedures and additional information. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) For a general review of allylic oxidation see: (a) Bulman Page, P. C.; McCarthy, T. J. In ComprehensiVe Organic Synthesis; Trost, B. M., Ed.; Pergamon: Oxford, U.K., 1991; Vol. 7, p 83. (2) For asymmetric allylic oxidations see: (a) Andrus, M. B.; Zhou, Z. J. Am. Chem. Soc. 2002, 124, 8806. (b) Andrus, M. B.; Lashley, J. C. Tetrahedron 2002, 58, 845. (c) Eames, J.; Watkinson, M. Angew. Chem., Int. Ed. 2001, 40, 3567. (3) Recent work: (a) Yu, J.-Q.; Corey, E. J. J. Am. Chem. Soc. 2003, 125, 3232. (b) Yu, J.-Q.; Corey, E. J. Org. Lett. 2002, 4, 2727. (c) Crich, D.; Zou, Y. Org. Lett. 2004, 6, 775. (4) Selected reviews: (a) Cainelli, G.; Cardillo, G. Chromium Oxidations in Organic Chemistry; Springer: Berlin, 1984. (b) Muzart, J. Chem. ReV. 1992, 92, 113. (5) Selected reviews: (a) Sharpless, K. B.; Verhoeven, T. R. Aldrichimica Acta 1979, 12, 63. (b) Sheldon, R. A.; Kochi, J. K. Metal-Catalyzed Oxidations of Organic Compounds; Academic Press: New York, 1981. (c) Metal-Catalyzed Selective Oxidations. In Peroxide Chemistry Mechanistic and PreparatiVe Aspects of Oxygen Transfer; Adam, W., Ed.; Wiley: Weinheim, 2000; p 301. (6) (a) Bien, S.; Segal, Y. J. Org. Chem. 1977, 42, 1685. (b) Noels, A. F.; Hubert, A. J.; Teyssie, P. J. Organomet. Chem. 1979, 166, 79. (c) Uemura, S.; Patil, S. R. Chem. Lett. 1982, 1743. (7) We have shown that Rh2(OAc)4 catalyzes the quantitative decomposition of TBHP: Doyle, M. P.; Terpstra, J. W.; Winter, C. H.; Griffin, J. H. J. Mol. Catal. 1984, 26, 259. (8) For Rh2(OAc)4-catalyzed oxidation of alcohols with TBHP see: Moody, C. J.; Palmer, F. N. Tetrahedron Lett. 2001, 43, 139. (9) Srinivasan, K.; Perrier, S.; Kochi, J. K. J. Mol. Catal. 1986, 36, 297. (10) Doyle, M. P.; Westrum, L. J.; Wolthuis, W. N. E.; See, M. M.; Boone, W. P.; Bagheri, V.; Pearson, M. M. J. Am. Chem. Soc. 1993, 115, 958. (11) Doyle, M. P.; Ren, T. In Progress in Inorganic Chemistry; Karlin, K., Ed; Wiley: New York, 2001; Vol. 49, p 113. (12) (a) Das, K.; Kadish, K. M.; Bear, J. L. Inorg. Chem. 1978, 17, 930. (b) Zhu, T. P.; Ahsan, M. Q.; Malinski, T.; Kadish, K. M.; Bear, J. L. Inorg. Chem. 1984, 23, 2. (13) Enedione 2 is a nonvolatile solid allowing rapid purification and determination of yield. (14) Anhydrous tert-butyl hydroperoxide in decane was obtained from Aldrich. (15) Longer reaction times did not increase yield. (16) For tert-butyl peroxy ethers as intermediates, see: refs 3a, 3b, and 6c. (17) Kornblum, N.; DeLaMare, H. E. J. Am. Chem. Soc. 1951, 73, 880. (18) See Supporting Information for optimization results. (19) Representative Procedure (0.1 mol % catalyst): A 25 mL flask equipped with a stirbar was charged with olefin (2.72 mmol, 100 mol %), CH2Cl2 (10 mL), K2CO3 (1.36 mmol, 50 mol %), and Rh2(cap)4 (0.0027 mmol, 0.1 mol %). The flask was sealed with a septum, allowing inclusion of air. An empty balloon was added to capture oxygen generated during the course of the reaction. To the mixture was added tert-butyl hydroperoxide (13.6 mmol, 5 equiv) in one portion via syringe from which the color of the solution immediately turned from light blue to deep red. Oxygen generation was observed inflating the balloon. After 1 h the solution was filtered over a short plug of silica gel to remove the catalyst. Column chromatography (SiO2) yielded the analytically pure compound. (20) (a) Kinzer, G. W.; Fentiman, A. F., Jr.; Foltz, R. L.; Rudinskyy, J. A. J. Econ. Entomol. 1971, 64, 970. (21) See Supporting Information for UV-visible spectrum. (22) (a) Cotton, F. A.; Walton, R. A. Multiple Bonds Between Metal Atoms; Wiley: New York, 1982; p 390. (b) Cotton, F. A.; Walton, R. A. Multiple Bonds Between Metal Atoms, 2nd ed.; Oxford: New York, 1993; p 475. (23) Isolated as a deep-red solid after chromatography. Submitting this complex (0.1 mol %) to the oxidation of 1 gave 2 in 53% under identical reaction conditions. See Supporting Information. We were unable to obtain a welldefined EPR signal of the oxidized Rh25+ complex. Efforts are underway to obtain a crystal structure of this complex. (24) (a) Bartlett, P. D.; Gunther, P. J. Am. Chem. Soc. 1966, 88, 3288. (b) Bartlett, P. D.; Guaraldi, G. J. Am. Chem. Soc. 1967, 89, 4799. (25) Bravo, A.; Bjorsvik, H.-R.; Fontana, F.; Liguori, L.; Minisci, F. J. Org. Chem. 1997, 62, 3849 and references therein. (26) (a) Kochi, J. K. Tetrahedron 1962, 18, 483. (b) Kochi, J. K.; Mains, H. E. J. Org. Chem. 1965, 30, 1862. (c) Minisci, F. Acc. Chem. Res. 1975, 8, 165. (d) Araneo, S.; Fontana, F.; Minisci, F.; Recupero, F.; Serri, A. J. Chem. Soc., Chem. Commun. 1995, 1399. (e) Minisci, F.; Fontana, F.; Araneo, S.; Recupero, F.; Banfi, S.; Quici, S. J. Am. Chem. Soc. 1995, 117, 226. (27) We have isolated 7 (where R ) Ph) from the oxidation of 1-phenylcyclohexene. Resubmitting 7 to the reaction resulted in quantitative conversion to the product shown in Table 1, entry 3. See Supporting Information for details.
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